The present invention relates generally to loudspeaker systems, and more particularly, to a protection circuit for a loudspeaker system that is responsive to the temperature of the loudspeaker driver mechanism for controlling loudspeaker operation.
It is common practice to provide protection circuits for loudspeaker systems in order to protect the loudspeaker driver mechanisms from excessive power levels. A common protection circuit, utilized for loudspeaker systems, employs a thermally sensitive resettable fuse with a power resistor in parallel. This protection circuit is wired in series with the loudspeaker driver mechanism. When the current passing through the thermally sensitive resettable fuse and the loudspeaker driver mechanism exceeds the current capacity of the thermally sensitive resettable fuse, the thermally sensitive resettable fuse opens, like a circuit breaker, and the power resistor becomes connected in series with the loudspeaker driver mechanism. The power resistor reduces the current through the protection circuit and the voltage on the loudspeaker driver mechanism, thus reducing the power applied to the loudspeaker driver mechanism and keeping it within a safe operating range.
Since loudspeaker driver mechanisms can handle more power when they are cold and less power when they are hot, it is advantageous to have a loudspeaker protection circuit that performs its function of protecting the loudspeaker system at progressively lower current levels as the loudspeaker driver mechanism progressively increases in temperature. The loudspeaker protection circuit in accordance with the invention performs this function.
In accordance with one aspect of the invention, a thermal protection circuit for a loudspeaker system is provided. The loudspeaker system may include a loudspeaker driver mechanism adapted for connection to a signal supply mechanism for providing an electrical signal to the driver mechanism. The thermal protection circuit includes an input that is adapted to receive an electrical signal from the signal supply mechanism and an output that is adapted to apply the electrical signal to the driver mechanism. The thermal protection circuit further includes a load device that is connected in series with the input and the output for reducing the power of the electrical signal before reaching the driver mechanism, and a first normally closed, thermally sensitive switch that is connected in parallel with the load device. The first switch is also thermally connectable to the driver mechanism such that heat generated by the driver mechanism is at least conductively transferable to the first switch. The first switch is changeable between a closed state wherein the load device is at least substantially bypassed, and an open state wherein the electrical signal is at least substantially directed through the load device when a temperature of the first switch is above a predetermined temperature and the electrical signal is above a predetermined signal level.
In accordance with a further aspect of the invention, a loudspeaker system includes a loudspeaker driver mechanism adapted for connection to a signal supply mechanism for providing an electrical signal to the driver mechanism, and a thermal protection circuit connected in series with the driver mechanism. The thermal protection circuit has a load device that is connected in series with the driver mechanism for reducing a power of the electrical signal before reaching the driver mechanism, and a first normally closed, thermally sensitive switch that is connected in parallel with the load device and thermally connected to the driver mechanism such that heat generated by the driver mechanism is at least conductively transferred to the first switch. The first switch is changeable between a closed state wherein the load device is at least substantially bypassed, to an open state wherein the electrical signal is at least substantially directed through the load device when a temperature of the first switch is above a predetermined temperature and the electrical signal is above a predetermined signal level.
In accordance with an even further aspect of the invention, a method of protecting a loudspeaker system against thermal overload includes connecting a load device in series with a loudspeaker driver mechanism and connecting a thermally sensitive switch across the load device. The thermally sensitive switch is changeable between a closed state wherein electrical current from a signal supply mechanism is at least substantially directed through the thermally sensitive switch, and an open state wherein the electrical current is at least substantially directed through the load device. The method further includes increasing the temperature of the thermally sensitive switch by conductively transferring heat from the driver mechanism to the thermally sensitive switch and by directing the electrical current through the thermally sensitive switch, and automatically changing the thermally sensitive switch from the closed state to the open state when the temperature of the thermally sensitive switch is above a predetermined temperature and the electrical current is above a predetermined current level.